Nowadays, local area networks (LAN) are widely applied to many areas e.g. home, office, school, laboratory and building. Many electronic devices such as personal computers, workstations, printers and servers are in communication with each other through the local area networks. Therefore, huge data are transmitted through LAN cables. Pulse transformers and common-mode filters are usually required at the interfaces between the LAN cables and the electronic devices.
Please refer to FIG. 1, an equivalent circuit diagram of a pulse transformer and a common-mode filter. The pulse transformer 11a provides direct-current blocking function between the physical side of the electronic device and the connected LAN cable. The pulse transformer 11a can keep transmission quality and reduce signal distortion of high speed digital signals.
At first, since there may exist potential difference (voltage) between the LAN cable and the electronic device, direct contact with the LAN cable may cause damage of the electronic device. Therefore, it is required to block direct current between the LAN cable and the electronic device, while alternating-current signals are allowed to be transmitted through the LAN cable. Furthermore, the physical side includes microelectronic circuits for signal modulation and demodulation and they are very sensitive to surge voltage/current. Impact resulting from the surge voltage/current may cause malfunction, damage even fire accident. In addition, surge voltage/current may occur because the LAN cables are exposed to the environment and bear temperature change, electric shock and wiring work. Effective protection against the surge can be achieved by using the pulse transformer 11a with direct-current blocking function.
Furthermore, in a differential pair, external electromagnetic interference affects both conductors of the differential pair so as to generate in-phase noises, e.g. common-mode noises. Moreover, direct-current interference introduced through a common ground or a power supply terminal may cause common-mode noises in the conductors. In addition to the pulse transformer 11a, the common-mode filter 12a can further suppress the common-mode noises. Furthermore, parasitic capacitance usually exists between both coils of the pulse transformer 11a. The common-mode noises at higher frequency entering one side of the pulse transformer 11a will be transmitted to the other side due to parasitic coupling effects. The common-mode filter 12a is useful to remove the high frequency noises.
Please refer to FIG. 1 again. The pulse transformer 11a and the neighboring common-mode filter 12a are arranged in a signal path. Both the pulse transformer 11a and the common-mode filter 12a are formed by winding coils around magnetic (or iron) units. Each of the pulse transformer 11a and the common-mode filter 12a has its own magnetic unit and coils. Although the pulse transformer 11a together with the common-mode filter 12a can achieve direct-current blocking and common-mode noise suppression, they indeed occupy much space and have adverse effect on size reduction. In addition, production cost thereof also increases.
Furthermore, if the two magnetic elements 11a and 12a are disposed too close, magnetic coupling between them occurs and results in interference and electric defect. In particular, high operation frequency, e.g. 100˜400 MHz in Gigabit Ethernet or higher transmission rate, makes the interference much worse. Therefore, it is difficult to balance electric property and size reduction of the magnetic component.
Accordingly, a composite magnetic component with reduced size while maintaining good electric and magnetic property is desired.